The present invention relates in a general way to an apparatus and method for aligning objects, and more particularly, for relatively aligning two objects.
Generally, in a semiconductor circuit manufacturing apparatus for manufacturing semiconductor devices, such as IC (Integrated Circuit) and LSI (Large-Scale Integrated Circuit), a mask and a wafer have to be aligned with each other prior to the wafer being exposed to the pattern formed in the mask.
FIG. 1 illustrates a prior art device for such an alignment operation, wherein measuring means 3L and 3R is provided to detect the relative deviation between the alignment marks LW and RW formed on the wafer 1 and the alignment marks LM and RM formed on the mask 2, respectively. If the deviations (.DELTA.XL, .DELTA.YL), (.DELTA.XR, .DELTA.YR) are within a predetermined tolerance, the alignment is deemed to have been accomplished. If, on the other hand, the deviation is larger than the tolerance, the wafer 1 and/or the mask 2 is shifted until the deviation falls within the tolerance.
For the purpose of increasing the accuracy of the alignment, a smaller tolerance is desirable, which, however, will require longer time to accomplish the tolerable alignment, since there can be an error in the correcting movement or shifting of the wafer stage or mask stage carrying the wafer 1 or mask 2, or in the measurement of the deviation between the wafer 1 and the mask 2. Accordingly, in order to achieve both alignment accuracy alignment speed, it is necessary to determine a proper tolerance corresponding to the desired alignment accuracy and alignment speed.
Here, it should be noted that ".DELTA.XL" and ".DELTA.YL" are the deviations between the left mark LM of the mask 2 and the left mark LW of the wafer 1, in X-direction and Y-direction, respectively. Similarly, ".DELTA.XR" and ".DELTA.YR" are the deviations between the right mark RM of the mask 2 and the right mark RW of the wafer 1, in X-direction and Y-direction, respectively.
Referring to FIGS. 2-5, the tolerance T employed in the prior art devices will be explained.
In the example shown in FIG. 2, the tolerance is defined by .vertline..DELTA.XL.vertline..ltoreq.T, .vertline..DELTA.YL.vertline..ltoreq.T, .vertline..DELTA.XR.vertline..ltoreq.T and .vertline..DELTA.YR.vertline..ltoreq.T, that is, the tolerance is defined by a regular square, as shown in the Figure.
FIGS. 5A and 5B illustrate states of a positional deviation between a pattern area 2A of the mask 2 and an area 1A of the wafer 1 to be exposed to the pattern. The area 1A of the wafer 1 corresponds to the entire surface of the wafer 1 in the case of a global alignment, while it corresponds to an area covered by a one shot exposure in the case of a divided exposing system, for example, as in a stepper.
The centers of the mask pattern area 2A and the wafer exposure area 1A are indicated by reference OM and OW; corners of the pattern area 2A and the exposure area 1A are designated by reference CM and CW; and the lengths of the edges of the pattern area 2A and the exposure area 1A are 2a and 2b, as shown in FIGS. 5A and 5B.
FIG. 5A indicates the relative position where there is no rotational deviation between the pattern area 2A and the exposure area 1A. If the deviation (.DELTA.XL, .DELTA.YL) is equal to (T, T), and (.DELTA.XR, .DELTA.YR) is equal to (T, T), the deviation between the pattern area 2A and the exposure area 1A, (.DELTA.X, .DELTA.Y) is (T, T) at any point in the pattern area 2A, because of no rotational deviation therebetween. If, however, there is rotational deviation between the pattern area 2A and the exposure area 1A, the maximum deviation occurs between the corner CM of the mask 2, and the corresponding corner CW of the wafer 1. The positional deviation, (.DELTA.XC, .DELTA.YC), between the point CM on the pattern area 2A and the point CM of the exposure area 1A is such that .DELTA.XC32 2T, and .DELTA.YC=T, when (.DELTA.XL, .DELTA.YL)=(T, -T); (.DELTA.XR, .DELTA.YR)=(T, T); and a =b. Therefore, the deviation does not satisfy the tolerance T at the point CW on the exposure area 1A.
In order to limit the deviation within the tolerance at the marginal areas of the pattern without changing the shape of the tolerance area (FIG. 2), the edges of the tolerance area of a rectangular shape as shown in FIG. 3 have to be T/2 in length, with the result that the area of the tolerance is reduced from 4T.sup.2 to T.sup.2. This will necessarily increase the time required for accomplishing the alignment.
FIG. 4 shows another example of the conventional tolerance area. In this example, the tolerance area is defined such that the deviations, (.DELTA.XL, .DELTA.YL) and (.DELTA.XR, .DELTA.YR) are within the tolerance when the following linear inequalities are satisfied: EQU .vertline..DELTA.XL.vertline.+.vertline..DELTA.YL.vertline..ltoreq.T EQU .vertline..DELTA.XR.vertline.+.vertline..DELTA.YR.vertline..ltoreq.T
Because of these inequalities of the first degree between .DELTA.X and .DELTA.Y, an alignment error at the marginal portions of the pattern area 2A in X-direction can be avoided even when there is a rotational deviation between the mask 2 and the wafer 1. In this case, if a=b is satisfied in FIG. 5, any point on the pattern area 2A satisfies the positional deviation .vertline..DELTA.X.vertline..ltoreq.T and .vertline..DELTA.Y.vertline..ltoreq.T, when the tolerance defined by FIG. 4 is satisfied. The tolerance area defined as shown in FIG. 4 is 2T.sup.2, and therefore, is twice as large as the tolerance area of FIG. 3, which will save the time required for the alignment. However, when the deviation is such that .DELTA.XL=2T/3, .DELTA.YL=2T/3, .DELTA.XR=2T/3, .DELTA.YR=2T/3, without rotational deviation, the deviation is less than the tolerance T at any point on the pattern area 2A, but this is rejected by either the tolerance defined by FIG. 3 or the tolerance defined by FIG. 4, with the result that an additional alignment operation is required. This means that an unnecessary alignment operation is required, and therefore, the alignment operation is made longer than necessary.
As described above, when it is desired that the positional deviations are within the tolerance of .vertline..DELTA.X.vertline..ltoreq.T and .vertline..DELTA.Y.vertline..ltoreq.T at any point on the pattern area 2A, the tolerance defined by FIG. 2 may cause a deviation over the tolerance at marginal areas of the pattern area 2A, while the tolerance defined by FIG. 3 or FIG. 4 can increase the alignment precision at the marginal areas of the pattern area, but it will require a longer alignment operation. That is, the tolerance of FIG. 2 will result in a lower yield, and the latter will result in a lower throughput.